The present invention generally relates to joining and bonding processes. The invention particularly relates to transient liquid phase bonding (TLPB) processes that include a bismuth-rich (Bi-rich) low-melting temperature phase (LTP).
Regulations increasingly prohibit the use of lead-based solders in the electronic interconnection and the electronic packaging industries. Pb-free solders intended to replace traditional eutectic PbSn solders previously investigated include SnAg, SnCu, SnAgCu, and SnZn solders. However, the development of high temperature Pb-free solders to substitute the conventional high lead solders (e.g., Pb-5Sn and Pb-5Sn-2.5Ag) are still in development.
Transient liquid phase bonding (TLPB) is a bonding process that joins substrate materials using an interlayer. On heating to a temperature above the melting point of the interlayer but below the melting point of the substrate materials, the interlayer melts and the interlayer element (or a constituent of an alloy interlayer) diffuses into the substrate materials, causing isothermal solidification. While holding the temperature above the interlayer melting point, interdiffusion and reaction with the substrate materials shifts the composition away from the starting interlayer composition, so solidification occurs at the process temperature to form a bond formed of a different composition (e.g., an intermetallic compound). If sufficient interdiffusion and intermetallic formation occurs, the joint may remain solid and strong well above the original melt process temperature, that is, the result of this process is preferably a bond that has a higher melting point than the bonding temperature. In general, the interlayer is formed of or includes a low melting temperature phase (LTP), such as a Sn or a Sn alloy, that is melted in the presence of a substrate material that is formed of or includes a high melting temperature phase (HTP), such as Ag, Cu, or Ni, such that the LTP is completely consumed by the formation of intermetallic compounds (IMC) upon heating.
While TLPB processes may provide benefits over other existing bonding technologies for certain applications, a so-called drop-in replacement for high Pb containing solder alloys currently used in electronics has not yet been produced. For example, previous investigations considered Cu—Sn—Bi TLPB systems wherein Sn—Bi alloys were isothermally solidified when in contact with Cu substrates. However, these investigations produced solid bonds with limited temperature capabilities, specifically being limited to operating temperatures below about 191° C., at which point the IMCs in the bond form a liquid.
In view of the above, it can be appreciated that there is an ongoing desire for improved systems and methods for replacing Pb-containing solders in the electronic industry that are capable of operating at temperatures comparable to the Pb-containing solders, for example, in excess of 200° C.